Survival problems are encountered at early stages of intensive fish rearing and antibiotics are widely used to remedy the situation. Probiotics may provide a potential alternative method to protect larvae from opportunistic and pathogenic bacteria and promote a balanced environment. This research work was undertaken to promote survival and larval development at early stages of Atlantic cod (Gadus morhua L.) rearing.The first part of the study was designed to search for probiotics to target this critical period in cod rearing. Potential probionts were selected from the natural microbiota of cod larviculture at an experimental station in Iceland, based on several in vitro tests. Our study demonstrated that 14% of screened bacteria (n=188) had antagonistic properties towards fish pathogens. The majority of the isolates were Gram-positive (81%), belonging to Firmicutes (69.2%) and Actinobacteria (11.5%) phyla based on 16S rRNA gene sequencing. Only six (3.2%) of 188 isolates could inhibit all three pathogens tested, Vibrio anguillarum, Aeromonas salmonicida ssp. achromogenes and Aliivibrio salmonicida. Differences observed in activity intensity and spectrum among inhibitory isolates emphasise the need to develop probiotic mixtures for efficient prophylactic methods.Successively, three bacterial strains were tested in vivo as a mixture added to the rearing water from an early postfertilisation stage in the experimental cod hatchery. Two of the added bacteria (Arthrobacter bergerei isolated from an algal concentrate and Enterococcus thailandicus from cod larval rearing water) were re-isolated in larvae at the end of the experiment, contributing to increased survival, growth, vitality and microbiota control. A different microbial rotifer treatment was attempted to introduce putative probiotics during larval first-feeding but was not successful and led to high larval mortality. Further, the dominant culturable microbiota was characterised as influenced by specific treatments applied during two spawning seasons. Current hatchery practices (ova disinfection and antibiotic treatment of unhealthy larvae) were evaluated, as well as specific putative probionts either applied to ova and larvae or to rotifers as the delivery vector to larvae. Further, virulence-related phenotypic traits, including haemolysin, siderophores and AHLs, were studied among bacterial isolates and related to larval survival. The results clearly demonstrate the influence of exogeneous feeding and treatments on larval gastrointestinal microbiota, and the role of bacteria in larval survival.During the third spawning season, the combined application of the two prospective probionts was validated by their addition via the rearing water from the prehatch stage till 28-dph larvae. This bacterial bathing treatment was compared to groups fed rotifers supplemented with a commercial probiotic (Remus®) and those untreated. Effects of treatments on the microbial load in rearing systems, larval growth, survival and development were evaluated. The results suggest that Arthrobacter and Enterococcus probionts affected the larval GI microbiota and contributed to growth, development and digestion, either directly or indirectly.Finally, an intervention study was conducted at an early cod juvenile stage to confirm the properties of the two probionts applied as a mixture or singly via dry feed (107-9 CFU g-1) to older fish for a shorter (28 days) or longer (55 days) period. This involved the development of probiotic feed, its viability and quality assessment under different storage conditions (-20, 4 and 15 °C), as well as the evaluation of effects on juvenile growth, survival, feed conversion ratio, microbial load of rearing water, juvenile gills and gut. Juveniles fed the Enterococcus-probiotic feed grew significantly faster and had significantly lower gut Vibrio counts than control juveniles (P<0.05). Overall, the results suggest that both probionts, especially the Enterococcus strain, modified the gut microbiota and contributed to enhanced juvenile growth and survival.